(a) Field of the Invention
The present invention relates to a surface light source device which is to be used as a back light for liquid crystal display units, etc., and more specifically to a surface light source device which uses a light transmitting member.
(b) Description of the Prior Art
The conventional surface light source device using a light guide consists, as shown in FIG. 1, of a light source 1, a light transmitting member 2 having an end surface 2a located in the vicinity of said light source, a diffusing plate 3 arranged on the front surface of the light transmitting member 2 and a reflecting surface arranged on the rear surface of the light transmitting member 2. This conventional surface light source device is adapted in such a manner that the light emitted from the light source 1 enters into the light transmitting member 2 through the end surface 2a thereof and sequentially transmitted to the other end surface 2b while being sequentially reflected by the front surface and the rear surface (reflecting surface). During the transmission, fractions of the light are scattered by the diffusing plate 3 and emerge as diffused light out of the diffusing surface 3. Since the diffusing plate 3 diffuses the light as described above, it allows the diffused light to emerge therefrom at a nearly constant luminance and is usable as a surface light source.
Such a surface light source device using a light transmitting member allows amount of light to be reduced as the portions of the diffusing plate are farther from the light source since the light is attenuated progressively as it travels through the light transmitting member. Accordingly, luminance on the front surface of the light transmitting member (the diffusing plate) is higher at the portions nearer the light source and lower at the portions farther from the light source.
As the conventional surface light source devices which have corrected the defect described above, there are known the surface light source devices using the linear patterns printed in milky white ink, paint or the similar material on the reflecting surfaces as shown in FIG. 2 and FIG. 3. For example, these patterns are formed as mesh-like patterns or parallel lines, as shown in FIG. 2 or FIG. 3, which are traced at narrow intervals on the side nearer the light source and at wider intervals toward the side farther from the light source so as to uniformalize luminance on the diffusing surface.
The conventional surface light source devices using the patterns traced on the reflecting surface of the light transmitting member or the reflecting plate cannot exhibit sufficient diffusing function and are insufficient in the effect to uniformalize luminance since these surface light source devices use linear patterns.
As another conventional surface light source device which has corrected this defect, there is known the surface light source device which is adapted so as to uniformalize luminance of the diffused light emerging from the diffusing plate 3, for example, by printing patterns in milky white ink or paint on the light transmitting member 2 so that the patterns are sparse in are in the vicinity of the end surface of incidence 2a and dense in area as they are farther from the end surface of incidence 2a as illustrated in FIG. 4.
As a further example of the conventional surface light source device, there is known the surface light source device which is adapted so as to increase amount of light by forming a second reflecting surface 5 on the end surface 2b located on the side opposite to the end surface of incidence 2a of the light guide, as shown in FIG. 5, so that the light having transmitted through the light transmitting member 2 and having reached the end surface 2b is reflected by the reflecting surface 5 and returns to the light guide 2.
The conventional example of this type also adopts, for uniformalizing the diffused light, patterns 6 which are formed by printing milky white ink or paint so as to have a diffusing function, and be at low density in area on the side of the end surface of incidence 2a and at high density in area on the side of the other end surface 2b as shown in FIG. 6.
The conventional surface light source device illustrated in FIG. 5 and FIG. 6 provides diffused light at luminance shown in FIG. 7. That is to say, this surface light source device has a constant luminance distribution from the end surface of incidence (at length zero) toward the other end surface (at length l), but enhances luminance at the portions close to the other end surface (the second reflecting surface 5) and has a defect that luminance cannot be said uniform at all the portions of the diffusing surface.
In order to correct this defect, there has been proposed the surface light source device which adopts the patterns illustrated in FIG. 8. The patterns of this conventional example are formed as dots having diameters which are small on the side of the end surface of incidence 2a, progressively larger as the portions of the reflecting surface are farther from the end surface of incidence 2a and smaller again in the vicinity of the other end surface (the second reflecting surface). In other words, the patterns 6 are arranged so that the density in area thereof is changed progressively from high to low from the end surface of incidence and then is changed from high to low again in the vicinity of the second reflecting surface. Accordingly, the diffused light emerging from the diffusing surface has a nearly uniform luminance distribution over the entire range from the end surface of incidence to the second reflecting surface.
This conventional surface light source device can provide diffused light uniform in luminance on the diffusing plate perpendicular to the light source 1. However, since it is sometimes necessary to equalize the length of the end surface of incidence 2a to the total length of the light source 1 due to restriction imposed on space, the conventional surface light source device has a defect in such a case that it allows luminance to be lowered at the portions of the diffusing plate close to both the ends of the end surface of incidence 2a under the influence produced by the electrodes attached to both the ends of the light source 1 (in the vicinity of the spots indicated by the reference symbol A).